1. Technical Field
The technology disclosed in this specification relates to a drive unit that drives a transistor to be driven, based on a drive signal.
2. Description of Related Art
An inverter or a voltage converter includes a device for switching a power transistor (transistor to be driven) for electric power conversion, between ON and OFF, based on a drive signal, namely, a drive unit that drives the power transistor.
Japanese Patent Application Publication No. 2009-111470 (JP 2009-111470 A) discloses a drive unit that is able to adjust the speed (which will be called “switching speed”) of switching a power transistor between ON and OFF, using a current mirror circuit. The drive unit disclosed in JP 2009-111470 A reduces the switching speed when the power transistor is not required to perform high-speed operation, so as to suppress or reduce switching noise of the power transistor.
The current mirror circuit may be of a type using N-channel transistors, or of a type using P-channel transistors. In the following, a current mirror circuit using N-channel transistors will be described by way of example. In the current mirror circuit, the gates of two transistors having the same gate threshold voltage are connected to each other, and a high-potential-side electrode of one of the transistors is connected to the gate of the transistor. A current source that causes constant current (reference current) to flow is connected to the high-potential-side electrode of the above-indicated one of the transistors. In the following, one of the two transistors of the current mirror circuit, to which the current source is connected, will be called “first transistor”, and the other transistor will be called “second transistor”.
When the current mirror circuit is used in the drive unit, one of high-potential-side electrode and low-potential-side electrode of the second transistor corresponds to an output terminal of the drive unit, to which the gate of the power transistor is connected. One of a high-voltage terminal that raises the voltage of the gate of the power transistor to the ON voltage, and a low-voltage terminal that reduces the gate voltage to the OFF voltage is connected to the other of the high-potential-side electrode and low-potential-side electrode of the second transistor. When the reference current flows, the first and second transistors of the current mirror circuit are placed in the ON state, and electric current is allowed to flow between the gate of the power transistor and the high-voltage terminal (or the low-voltage terminal). If the reference current is stopped, and the gate voltage of the transistors of the current mirror circuit is lowered, the first and second transistors are placed in the OFF state, and the gate of the power transistors is disconnected from the high-voltage terminal (or the low-voltage terminal).
In the current mirror circuit, electric current proportional to the reference current flows through the second transistor. If the reference current is reduced, the current that flows through the second transistor when it is in the ON state is reduced, and the rate of change of the gate voltage of the power transistor is reduced. As a result, the switching speed of the power transistor is reduced. The drive unit of JP 2009-111470 A adjusts the switching speed of the power transistor (transistor to be driven), by adjusting the magnitude of the reference current.